The Sandstone and Limestone of the Nakanosawa Formation

Abstract

The sedimentary rocks of the upper Jurassic Nakanosawa formation were studied in this paper., They are classified into following ones., 1) A (dark gray) sandstone., This is abundant in matrix minerals and biotite flakes, and the diameters of the sand grains are small., : (Graywacke and subgraywacke after PETTIJ0HN'S classification)., 2) B (white) sandstone., : This is characterized by the large diameters of its sand grains, and contains a tew matrix mineral and biotite flake., (Orthoquartzite after PETTIJ0HN'S classification)., 3) C (calcareous) sandstone., : This yields abundant lime matrix, and its sand greins show large diameters., 4) Oolitic limestone., 5) Black, dense limestone., The B sandstone was the product under the condition of stronger water current than the A sandstone., The C sandstone and oolitic limestone were formed, when the depositional environments showed the relative increase of the deposition of lime, under rather strong water current., The relation between the volume percentage and the diameter of sand grains of the sandstones, and those of ooid of the oolitic limestone were examined, and it is concluded that the depositional condition of lime matrix of oolitic limestone was the same with that of calcareous sandstone, and sericite-like mineral as matrix of the A and B sandstone were washed away more easily than lime as matrix of the calcareous sandstone and oolitic limestone under a certain current., "Composite" ooids, composed of small simple ooids, fossil remains, and lime matrix, were formed when they rolled about on the sea bottom., The sandstones of the Tochikubo formation and the Tomisawa formation, which are overlain and underlain respectively by the Nakanosawa formation, have a good deal of feldspar grains, and the lower part of the Nakanosawa formation contains more feldspar grains than the upper part., The writer concluded that the backgrounds of the depositional basin rose before the deposition of the Tochikubo formation, and the Nakanosawa age was that of planation, and after the deposition of the Nakanosawa formation the backgrounds rose again., The limestone of the Nakanosawa formation was formed by the decrease of the deposition of terrigenous materials in the age of planation., The deposition of the lower part of the Koike limestone (the uppermost part of the Nakanosawa formation), mainly composed of oolitic limestone, was under the influence of rather strong water current, and the upper part of the lime stone, consisting of much black, dense limestone and oolitic limestone, was formed at deeper sea bottom than the lower part.,